This invention relates to ultra-reliable pressure sensor assemblies for use under adverse conditions including high vibration and corrosive fluids.
As disclosed in U.S. Pat. No. 5,935,189, for example, pressure sensors may be employed to sense the exhaust pressure of internal combustion engines. These exhaust gases contain moisture and highly corrosive gases. It has previously been proposed to use semiconductor pressure sensors with a very thin layer of glass passivation on the surface thereof. In addition, gel has been employed to further protect against corrosion, see U.S. patent application Ser. No. 09/204,326. However, as discussed below, these sensors have been subject to corrosion degradation despite the glass coating, with or without the additional gel.
Incidentally, sensors per se are disclosed in U.S. Pat. Nos. 5,929,498 and 5,578,843, as well as in U.S. patent application Ser. No. 09/704,376, filed Nov. 2, 2000 (Docket 410005-144), all of which are assigned to the assignee of the present invention. The specification and drawings of these prior patents and patent application are hereby incorporated into this application by reference. It is also noted that vacuum deposited polymeric coatings have previously been used to protect electronic semi-conductive chip assemblies.
In accordance with one aspect of the invention, it has been discovered that, in the adverse environment of automobile exhaust, gases, semi-conductor pressure transducers, even with glass passivation and gel protection, are subject to significant degradation due to the penetration of moisture through the gel, over time. More specifically, when the leads are sonically bonded to the conductive pads on the chips, tiny cracks may be formed which permits entry of corrosive gases and moisture. Also, pin hole openings may be present in thin glass layers.
To overcome this problem it has been determined that additional effective anti-corrosion protection may be obtained through the use of an additional vacuum deposited polymeric coating in addition to a glass layer and gel. Accordingly, a sensitive, reliable, corrosion proof pressure sensor system for the adverse vibration and corrosive intensive environment of internal combustion engines may be realized by the use of a semi-conductive pressure diaphragm and associated electronics mounted on a substrate, with conductive wire bonding pads and sonically welded leads, and with glass passivation over the substrate, an additional gel, and a vapor deposited polymeric coating being provided to insure complete protection of the semi-conductive components and the lead bonding areas. Preferably, the semi-conductive material is protected against oxidation by the thin glass layer, the vacuum formed polymeric layer is deposited, and finally a layer of gel overlies the subassembly.
In accordance with a feature of the invention, a corrosion proof pressure transducer system has a pressure transducer including a chip having a variable capacitor including a diaphragm, preferably a semi-conductive diaphragm, and conductive pads, thereon, together with additional electronic circuitry coupled to the variable capacitor and pads, with the chip having glass passivation thereon, wires sonically bonded to the pads, and a vacuum deposited polymeric coating extending over the chip including the pads, wire connections and leads; and with gel being optionally provided to overly the chip, either before or after the deposition of the polymeric coating.
To summarize, it would normally be expected that the glass passivation layer and a layer of gel would be adequate to preclude corrosion degradation of the pressure sensor. However, it has been determined that, with (1) cracks produced by wire bonding, (2) the vibration present in an automobile exhaust system, together with (3) the very corrosive exhaust gases and moisture, the further protection as outlined above is desirable to provide a long life, reliable and dependable pressure sensor.
Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and the accompanying drawings.